Terephthalic acid is the starting material for polyethylene terephthalate (PET), which is the principal polymer used to manufacture polyester fibers, polyester films, and resins for bottles and like containers. Crude terephthalic acid (CTA), which is produced by the direct oxidation of p-xylene by oxygen, must be purified to be useful for manufacturing high quality and low color PET, which is produced by a condensation reaction between purified terephthalic acid (PTA) and ethylene glycol. Even after initial purification, however, terephthalic acid typically contains residual amounts of colored contaminants. It is highly desirable to reduce the concentration of such remaining colored impurities prior to using the acid to produce PET.
It is known that terephthalic acid can be purifed by subjecting the crude acid to hydrogenation in the presence of a noble metal catalyst, as described, for example, in Meyer, U.S. Pat. No. 3,584,039, Stech et al., U.S. Pat. No. 4,405,809, and Pohlmann, U.S. Pat. No. 3,726,915, the disclosures of which are incorporated herein by reference. Additional descriptions of the purification of aromatic dicarboxylic patents by catalytic hydrogenation processes are included in the following patents, the disclosures of which are incorporated herein by reference:
Shigematsu et al., U.S. Pat. No. 6,455,731, discloses a three-step process for producing high-purity aromatic acids: preparing an amine salt by the reaction of crude acid with an amine, continuously decomposing the amine salt to form a slurry of crystallized acid, and separating crystals of the acid from the slurry.
Sargent et al., U.S. Pat. No. 3,151,154, discloses a process for decolorizing phthalic acids (or salts thereof) prepared by the nitric acid oxidation of xylenes or toluic acids that comprises hydrogenating the crude oxidation product in the presence of a catalyst, acidifying the mixture with a strong acid, and recovering the phthalic acid that crystallizes from solution.
Meyer, U.S. Pat. No. 3,584,039 discloses a process for preparing fiber-grade terephthalic acid by catalytic hydrogenation of a solution of impure terephthalic acid, followed by separation of the catalyst and crystallization of the purified acid, impurities being retained in the aqueous mother liquor.
The following patents, all issued to Puskas et al., relate to the purification of terephthalic acid by hydrogenation in the presence of a palladium catalyst formed by contacting a porous carbon support with an aqueous solution of a palladium salt and an amine in the presence of an organic carboxylic acid: U.S. Pat. No. 4,394,299, U.S. Pat. No. 4,415,479, and U.S. Pat. No. 4,467,110.
The color level of purified terephthalic acid is generally evaluated either by measuring the optical density of a solution of the purified terephthalic acid or by determining the b* value of the solid purified terephthalic acid. The optical density of purified terephthalic acid is measured as the absorbance of light at 340 nm nanometers in its basic solution in a solvent such as sodium hydroxide or ammonium hydroxide.
The measurement of the b* value of a solid on the Hunter Color Scale is described in Hunter, The Measurement of Appearance, Chapter 8, pp, 102–132, John Wiley & Sons, N.Y., N.Y. (1975), and in Wyszecki et al., Color Science, Concepts and Methods, Quantitative Data and Formulae, 2d Ed., pp. 166–168, John Wiley & Sons, N.Y., N.Y. (1982). A spectrophotometer can be used to measure the spectrum of visible light reflected from a sample of the solid acid, from which spectrum the b* value of purified terephthalic acid can be determined.
The use of b* values as a measure of quality for aromatic dicarboxylic acids purified by catalytic hydrogenation is described in, for example, Schroeder et al., U.S. Pat. No. 4,892,972, Schroeder, U.S. Pat. No. 4,937,378, Bartos et al., U.S. Pat. No. 5,616,792, and Rosen et al., U.S. Pat. No. 5,756,833, the disclosures of which are incorporated herein by reference.